The present invention relates in general to power supplies, and more particularly to a switching power supply.
Rectified and filtered alternating current line voltage has been converted to a direct current output voltage through the repetitious opening and closing of a switch in series with the primary winding of a transformer. When the switch was closed, the current flow in the primary winding of the transformer increased and energy was stored in the core of the transformer. During this interval of time, a rectifier in the secondary winding circuit of the transformer was reversed biased and non-conducting. When the switch was opened, the action of the inductance of the primary winding caused the voltage across the primary winding to increase in magnitude above the rectified and filtered line voltage. Thereupon, the rectifier in the secondary winding circuit conducted to provide a direct current output voltage. The direct current output voltage was a function of the energy stored in the core of the line transformer.
With the advent of computers, it became important to provide a power supply that maintained a relatively constant maximum power output at various line voltages. It has been known that line voltages and line frequencies may vary from country to country. Yet, the power requirements for the storage of data in memories must remain within fixed limits. The need for a constant power output was apparent. Under fault conditions, additional power over the rated power may result in damage to the wiring of computers. Failure of the power supply to properly handle power in excess of the rated power may cause the power supply to fail.
In typical flyback power supplies, the power supply was protected against overloading by limiting the peak current to a known value. This arrangement has not been satisfactory. While the load current remained constant, the rectified and filtered a.c. line voltage varied. Increases in the line voltage resulted in the increases in the power output. Such systems were not economical to manufacture.